1. Industrial Useful filed
This invention relates to a lock-up device for a so-called pressure piston type torque converter suitable for a heavy duty vehicle for use in a construction machine, in which an inside pressure of the torque converter is low and its transmission torque is large when a lock-up action is turned on.
2. Prior Art
A torque converter equipped with a lock-up device has conventionally been used for the purpose of eliminating a slippage of fluid to improve fuel consumption and prevent an engine torque from decreasing in a coupling zone of the torque converter.
With advance in automatic transmissions, the engagement rate of the lock-up device increased and the requirements for better fuel consumption increased, so that use of lock-up devices becomes necessary even in a torque converter zone wherein engine torque is multiplied. The lock-up device is used in all the speed change stages so that an application frequency of the lock-up device may increase by a large margin. Further, the above lock-up action can be easily controlled by electronic means. In case, for example, when speed is changed from one lock-up state to another state, it is sometimes required to temporarily release the lock-up state at the time of speed change. The lock-up action is executed even when a difference of rotation speeds is large between the engine and a turbine forming an output of the torque converter, so that the work volume of friction material of the lock-up device increases and need for heavy duty operation rises. Moreover, shock torque will seldomly be generated in the lock-up action when the rotation speed difference between engine and turbine is small, but will be more readily generated when the difference is large.
In conventional lock-up devices; one system has been employed for passenger cars, wherein a friction material bonded to a piston installed at a turbine side is pressed on a front cover, and another system has been employed for heavy duty industrial construction machines, wherein a piston installed at the front cover side pinches a disc installed at the turbine side.
In the lock-up device for heavy duty application, however, a pressure surface of the piston is formed into a single-stage shape so that shock will be generated when pressure oil is supplied thereto unless such shock is counteracted. In order to eliminate this shock, a booster consisting of a control valve and an accumulator must be installed and the entire mechanism will inevitably become complicated in both mechanical and electrical aspects and increase manufacturing cost.